Method of manufacturing a body having anisotropic, permanent magneticproperties

ABSTRACT

A METHOD OF MANUFACTURING SINTERED M5R MAGNET BODIES (M = CO, POSSIBLY COMBINED WITH FE, NI, CU, AND R = ONE OR MORE OF THE RARE EARTHS AND/OR TH). STARTING MATERIALS ARE TWO CASTINGS ONE OF THE ATOMIC RATIO M:R &lt; 5 AND ONE OF M:R &gt; 5. THEY ARE PULVERIZED, ORIENTED, COMPRESSED AND SINTERED IN AN ATOMOSHERE WHICH IS VERY POOR IN OXYGEN AND WATER VAPOUR.

United States Patent C Int. Cl. B22f 1/00; Hillf 1 /08 US. Cl. 148-103 4 Claims ABSTRACT OF THE DISCLOSURE A method of manufacturing sintered M R magnet bodies (M=Co, possibly combined with Fe, Ni, Cu, and R=one or more of the rare earths and/or Th). Starting materials are two castings one of the atomic ratio M:'R 5 and one of M:R 5. They are pulverized, oriented, compressed and sintered in an atmosphere which is very poor in oxygen and water vapour.

The invention relates to a method of manufacturing a body having anisotropic, permanent magnetic properties, the constituent essential to said properties being a compound having a hexagonal crystal structure, the existence range of which forms one assembly with the existence range of the compound M R occurring in the system M-R, where M is Co or a combination of Co with one or more of the elements Fe, Ni and Cu, and R is one or more of the elements of the rare earths and/or Th, by sintering a powder which consists of a compound of M and R.

In this connection the element Y is deemed to be included in the elements of the rare earths.

Such a method is known from the published Dutch patent application number 6807894. The resulting magnetic body is magnetically anisotropic when the powder particles, before being sintered, are oriented in a magnetic field.

It is also known to manufacture permanent magnets built up from the compound M R by compressing M R in powder form. In Philips Technical Review 29 (1968), p. 336 et seq. it is described, for example, how to manufacture in this manner an SmCo magnet having very good permanent magnetic properties.

Compared with the method in which M R powder is compressed to a magnet body, the sintering method has a few advantages. First of all, the sintering method is much more economic for series production of M R magnets: no heavy press is needed, for example. Secondly, the coercive force of the sintered M R magnets is considerably higher than that of compressed M R magnets having the same M and R. Thirdly, the coercive force of a sintered M R magnet may be approximately constant whereas that of a compressed M R magnet decreases reduces as a function of timethe so-called ageing.

However, when, for example, SmCo powder having a hexagonal crystal structure and a grain size 100p. is compressed and then sintered at a temperature between 800 C. and 1250 C., if desirable in the presence of a suitable gettering substance, for example Th, a magnet body thus formed turns out to have a very low coercive force, for example 100 oe. This is a result of the fact that, in addition to SmCo the compound 5111 00 (for example 70-90%) has formed as a compound of Sm and Co after sintering, which compound has poor permanent magnetic properties.

An obvious measure which can result in an improvement of the magnetic properties of the above-mentioned ice magnet body is that not SmCo powder is used as the starting material but the starting material chosen is a powder mixture which is richer in Sm, for example, a mixture in which the atomic ratio Sm:Co=l:3.8.

It has been found that after sintering such a mixture, Sm Co is no longer present any longer in the sintered body. The only Sm--Co compound occurring is found to be the desired SmCo It is to be noted that sometimes a small quantity of Sm Co is found in addition to SmCo which, however, is not harmful since Sm Co also has permanent magnetic properties. A coercive force of 36; 000 Oe. was measured with a sintered body obtained in this manner (sintering is generally carried out in the presence of, for example, TK as a gettering substance).

However, the retentivity B, measured at the sintered body was found to be comparatively low, namely approximately 6000 G. As is known, a lower B results in a lower energy product (BH) of the ultimate permanent magnet.

The cause of this comparatively low Br has surprisingly been found to be the presence of a comparatively high quantity (40% by vol.) of Sm O in the sintered body. This is not surprising because this means that very high weight percentages of oxygen and/or water, namely 1.6% by weight of 0 or 1.7% by weight of H 0, must have been present in the powder.

Once it is known that much Sm O is present in the sintered body, known methods can be used with which the formation of Sm O during sintering is prevented. This can be carried out, for example, by grinding the starting mixture under dried toluene in order to obtain an anhydrous powder. This may be done alternatively by annealing the powder, possibly in the presence of an oxygen getter, for example, Ca, so as to remove the oxygen therefrom. Said known methods turned out to give unsatisfactory results. The desirable result is obtained indeed when the castings from which was startedif desirable, after first having coarsely ground them-were pulverized in an atmosphere protecting against oxidation, for example, in a rare gas atmosphere, in which at most ppm. of water vapour and/or oxygen or a mixture thereof was present. It was found that the loss of Sm as a result of the formation of Sm Co during sintering was restricted considerably. In a body formed by sintering a powder of the composition SmCo in the last-mentioned conditions, only 1 to 2% of Sm O turned out to occur in addition to the magnetic SmCo It was found, however, that the gain in retentivity B and hence in (BH) of the ultimate permanent magnet, which was expected after elimination of the nonmagnetic Sm O compound from the sintered body, did not occur. The cause hereof appeared to be that the increased density of the permanent magnetic SmCo which usually occurs after sintering said powder, did not occur now, that is to say after grinding and sintering in the said atmosphere which is poor in oxygen and water-vapour.

It has surprisingly been found now that, when the starting powder is ground, oriented, compressed and sintered in oxygen-free and water vapour-free conditions, a higher density of the SmCo can nevertheless be obtained when during sintering a starting material issued consisting of a powdered mixture composed of a component which is rich in Co and a component which is poor in Co. A higher B is the result of said higher density. When in particular both said components can be readily oriented in a magnetic field, the B increases even further.

The method according to the invention is characterized in that first castings are manufactured, at least one having an atomic ratio M:R 5 and one having an atomic ratio M:R 5, which are pulverized and mixed in an atmosphere protecting against oxidation and containing more than 100 ppm. of oxygen and/or water vapour,

that in the same atmosphere said powder is then oriented in a magnetic field, compressed and sintered between 800 C.-1250 C.

Essential to the invention thus is that grinding, orientation, compression and sintering is carried out in an atmosphere which is poor in oxygen and water vapour while the starting material does not consist of one compound but of at least two compounds in such a ratio that finally after sintering the desired M R compound is obtained.

A preferred embodiment of the method according to the invention is characterized in that both the powder of the atomic ratio M:R 5 and that of the atomic ratio M:R 5 can readily be oriented in a magnetic field.

When the powder particles have been oriented magnetically, the B which has increased already due to the higher density of the SmCo in the sintered body, will increase even further.

A further preferred embodiment of the method according to the invention is characterized in that the atmosphere protecting against oxidation contains less than 5 p.p.m. of oxygen and/ or water vapour.

The invention furthermore comprises bodies having anisotropic, permanent magnetic properties manufactured by any of the above-mentioned methods.

The invention will now be described with reference to the following specific examples.

EXAMPLES (I) Two castings, the atomic ratios SmzCo were (1:5 .4) and (2:7), respectively, were manufactured by melting and then solidifying. It is to be noted that just below the solidification point of the melt in which Sm:Co=1:5 .4, a compound SmCo is formed which is single phase with the compound SmCo The further cooling of said compound without special precautions being necessary occurs already so rapidly that said single phase is maintained and that no splitting up into SmCo and Sm Co takes places.

The castings are coarsely ground. Of these coarsely ground fractions, 3.0 gms. of SmCo and 1.5 gms. of Sm CQ were ground in a mortar until the average diameter of the particles was less than 30a. The grinding process in the mortar took place in a so-called glove box in which an Ar atmosphere prevailed which contained approximately 1 p.p.m. of oxygen and approximately 1 p.p.m. of water vapour.

The ground powder, still in the glove box, was trans ferred to a rubber pouch which was evacuated, closed and taken out of the glove box. Said pouch was then brought in a magnetic field of 40,000 oe. in which the powder was oriented. The powder was compressed to a density of 82% under a hydrostatic pressure of 20 kb. The pouch containing the thus oriented and compressed powder was again transferred to the glove box in which the powder, together with Th which during the sintering process is operative as a getter, was wrapped in a molybdenum foil which was enclosed in an iron capsule. The capsule was then annealed at 1120 C. for 30 minutes at which the powder was sintered. After sintering, the sintered body turned out to consist of SmC0 for approximately 99% and of Sm O for approximately 1%. The density was found to have increased to 95% during sintering.

After magnetising the sintered body, the following magnetic values of the resulting permanent magnet were measured:

H =36,000 oe. B,=8200 G (BH) =16.1 MG oe.

(II) Starting material were two castings of the atomic ratio Sm:Co=1:5 .4 and 1:3.8, respectively. The last-mentioned casting consisted of the compound Sm Co and SmCo (approximately 70% by volume and approximately 30% by volume, respectively), so that the powder mixture which was finally sintered consisted of SmCo powder, Sm Co powder and SmC0 powder, and that in the weight ratio: 29.10z42.

The conditions of grinding, orientation and compression were equal to those described in Example I.

After sintering, approximately 99% of SmCo and approximately 1% of Sm O were found to be present again in the sintered body, while due to the sintering the density had increased from 821% to 94%.

The measured magnetic values of the permanent magnet were:

H =32,000 oe. B =7200 G (BH) =16.0 MG oe.

(III) Starting material was a powder mixture of 1.2 gms. of SmCo and 2.6 gms. of Srn Co, which had been obtained by grinding in a glove box which contained 600 p.p.m. of oxygen and/or water vapour. After orientation and compression to a density of 82%, an iron capsule containing the powder with the gettering substance in a molybdenum foil was annealed at 1120 C. for 30 minutes. The resulting sintered body obtained approximately 85% of SmCo and approximately 15% of Sm O and had a density of The measured magnetic values of the permanent magnet were:

H =35,000 oe. B =7200 G (BH) =12.0 MG 0e.

Compared with the results of Example I, the detrimental influence of the presence of the oxygen and/or water vapour during grinding is obvious from these results.

(IV) Starting material was a powder of the composition SmCo The powder had been obtained by grinding in a glove box as in Example I and it was oriented and compressed to a density of 82% under the same conditions. Sintering was then carried at 1100 C. The sintered body was found to contain approximately 99% of SmCo and approximately 1% of Sm O During sintering the density had not increased and was still 82%.

Of the permanent magnet manufactured from this sintered body, the following magnetic values were measured.

H =35,000 0e. B =7200 G (BH) =11.8 MG oe.

From these results it appears that when the starting powder does not consist of a mixture of powders with Co:Sm 5 and Co:Sm 5, the desirable increase in density does not occur and hence the B, does not reach the desirable high value.

What is claimed is:

1. A method of manufacturing a body having anis0- tropic, permanent magnetic properties, the component essential to the said properties being a compound having a hexagonal crystal structure, the existence range of which forms one assembly with the existence range of the compound M R occurring in the system MR, where M is an element selected from the group consisting of Co and a combination of Co with at least one of the elements Fe, Ni and Cu, and where R is selected from the group consisting of at least one of the rare earths, Th, and Th in combination with one of the rare earths comprising the steps of compacting and sintering a powder which consists of a compound of M and R to form castings, at least one of which has an atomic ratio M:R 5 and another has an atomic ratio M:R 5, pulverizing and mixing said castings in an atmosphere protecting against oxidation and containing 100 p.p.m. each of oxygen and water vapour in proportions forming a powder which after sintering forms a body consisting essentially of the compound M R, orienting said powder in a magnetic field while in said atmosphere, and thereafter compacting and sintering said powder between 800 C. and 1250 C. to form said body.

2. A method as claimed in claim 1, wherein both the powder of the atomic ratio M:R 5 and that of the atomic ratio M:R 5 can be readily oriented in a magnetic field.

- 5 3. A method as claimed in claim 1, wherein the atmos- 3,546,030 12/1970 Buschow et a1. 148-103 X phere protecting aginst oxidation contains less than 5 3,591,428 7/1971 Buschow et a1. 14831.57 p.p.m. each of oxygen and water vapour. 3,560,200 2/ 1971 Nesbitt et a1. 75122 4. A method as claimed in claim 3, wherein the starting powder consists of a mixture of Sm Co and SmCo 5 DEWAYNE RUTLEDGE, Primary Examiner References Cited G. K. WHITE, Assistant Examiner UNITED STATES PATENTS U.S. Cl. X.R.

3,424,578 1/1969 Strnatet-al 7s-213 75-200, 213, 224; 148-101 3,425,828 2/1969 Cape 7s--200 

